5,837 research outputs found
Neutron stars with hyperon cores: stellar radii and EOS near nuclear density
The existence of 2 Msun pulsars puts very strong constraints on the equation
of state (EOS) of neutron stars (NSs) with hyperon cores, which can be
satisfied only by special models of hadronic matter. The radius-mass relation
for these models is sufficiently specific that it could be subjected to an
observational test with future X-ray observatories. We want to study the impact
of the presence of hyperon cores on the radius-mass relation for NS. We aim to
find out how, and for which particular stellar mass range, a specific relation
R(M), where M is the gravitational mass, and R is the circumferential radius,
is associated with the presence of a hyperon core.
We consider a set of 14 theoretical EOS of dense matter, based on the
relativistic mean-field (RMF) approximation, allowing for the presence of
hyperons in NSs. We seek correlations between R(M) and the stiffness of the EOS
below the hyperon threshold needed to pass the 2 Msun test. For NS masses
1.013km, because of a very stiff pre-hyperon segment of
the EOS. At nuclear density, the pressure is significantly higher than a robust
upper bound obtained recently using chiral effective field theory.
If massive NSs do have a sizable hyperon core, then according to current
models the radii for M=1.0-1.6 Msun are necessarily >13km. If, on the contrary,
a NS with a radius R<12 km is observed in this mass domain, then sizable
hyperon cores in NSs, as we model them now, are ruled out. Future X-ray
missions with <5% precision for a simultaneous M and R measurement will have
the potential to solve the problem with observations of NSs. Irrespective of
this observational test, present EOS allowing for hyperons that fulfill
condition M_max>2 Msun yield a pressure at nuclear density that is too high
relative to up-to-date microscopic calculations of this quantity.Comment: 10 pages, 10 figures, published in A&
Rotating neutron stars with exotic cores: masses, radii, stability
A set of theoretical mass-radius relations for rigidly rotating neutron stars
with exotic cores, obtained in various theories of dense matter, is reviewed.
Two basic observational constraints are used: the largest measured rotation
frequency (716 Hz) and the maximum measured mass (). Present status
of measuring the radii of neutron stars is described. The theory of rigidly
rotating stars in general relativity is reviewed and limitations of the slow
rotation approximation are pointed out. Mass-radius relations for rotating
neutron stars with hyperon and quark cores are illustrated using several
models. Problems related to the non-uniqueness of the crust-core matching are
mentioned. Limits on rigid rotation resulting from the mass-shedding
instability and the instability with respect to the axisymmetric perturbations
are summarized. The problem of instabilities and of the back-bending phenomenon
are discussed in detail. Metastability and instability of a neutron star core
in the case of a first-order phase transition, both between pure phases, and
into a mixed-phase state, are reviewed. The case of two disjoint families
(branches) of rotating neutron stars is discussed and generic features of
neutron-star families and of core-quakes triggered by the instabilities are
considered.Comment: Matches published version. Minor modifications and reference adde
Consequences of a strong phase transition in the dense matter equation of state for the rotational evolution of neutron stars
We explore the implications of a strong first-order phase transition region
in the dense matter equation of state in the interiors of rotating neutron
stars, and the resulting creation of two disjoint families of neutron-star
configurations (the so-called high-mass twins). We numerically obtained
rotating, axisymmetric, and stationary stellar configurations in the framework
of general relativity, and studied their global parameters and stability. The
instability induced by the equation of state divides stable neutron star
configurations into two disjoint families: neutron stars (second family) and
hybrid stars (third family), with an overlapping region in mass, the high-mass
twin-star region. These two regions are divided by an instability strip. Its
existence has interesting astrophysical consequences for rotating neutron
stars. We note that it provides a natural explanation for the rotational
frequency cutoff in the observed distribution of neutron star spins, and for
the apparent lack of back-bending in pulsar timing. It also straightforwardly
enables a substantial energy release in a mini-collapse to another neutron-star
configuration (core quake), or to a black hole.Comment: 9 pages, 7 figures, Astronomy and Astrophysics accepte
Neutron star radii and crusts: uncertainties and unified equations of state
The uncertainties in neutron star (NS) radii and crust properties due to our
limited knowledge of the equation of state (EOS) are quantitatively analysed.
We first demonstrate the importance of a unified microscopic description for
the different baryonic densities of the star. If the pressure functional is
obtained matching a crust and a core EOS based on models with different
properties at nuclear matter saturation, the uncertainties can be as large as
for the crust thickness and for the radius. Necessary
conditions for causal and thermodynamically consistent matchings between the
core and the crust are formulated and their consequences examined. A large set
of unified EOS for purely nucleonic matter is obtained based on 24 Skyrme
interactions and 9 relativistic mean-field nuclear parametrizations. In
addition, for relativistic models 17 EOS including a transition to hyperonic
matter at high density are presented. All these EOS have in common the property
of describing a star and of being causal within stable NS. A span
of km and km is obtained for the radius of, respectively,
and star. Applying a set of nine further
constraints from experiment and ab-initio calculations the uncertainty is
reduced to km and km, respectively. These residual uncertainties
reflect lack of constraints at large densities and insufficient information on
the density dependence of the EOS near the nuclear matter saturation point. The
most important parameter to be constrained is shown to be the symmetry energy
slope which exhibits a linear correlation with the stellar radius,
particularly for masses . Potential constraints on , the
NS radius and the EOS from observations of thermal states of NS are also
discussed. [Abriged]Comment: Submitted to Phys. Rev. C. Supplemental material not include
Fermionic characters for graded parafermions
Fermionic-type character formulae are presented for charged
irreduciblemodules of the graded parafermionic conformal field theory
associated to the coset . This is obtained by counting the
weakly ordered `partitions' subject to the graded exclusion principle.
The bosonic form of the characters is also presented.Comment: 24 p. This corrects typos (present even in the published version) in
eqs (4.4), (5.23), (5.24) and (C.4
New bases for a general definition for the moving preferred basis
One of the challenges of the Environment-Induced Decoherence (EID) approach
is to provide a simple general definition of the moving pointer basis or moving
preferred basis. In this letter we prove that the study of the poles that
produce the decaying modes in non-unitary evolution, could yield a general
definition of the relaxation, the decoherence times, and the moving preferred
basis. These probably are the most important concepts in the theory of
decoherence, one of the most relevant chapters of theoretical (and also
practical) quantum mechanics. As an example we solved the Omnes (or
Lee-Friedrich) model using our theory.Comment: 6 page
Thermalisation time and specific heat of neutron stars crust
We discuss the thermalisation process of the neutron stars crust described by
solving the heat transport equation with a microscopic input for the specific
heat of baryonic matter. The heat equation is solved with initial conditions
specific to a rapid cooling of the core. To calculate the specific heat of
inner crust baryonic matter, i.e., nuclear clusters and unbound neutrons, we
use the quasiparticle spectrum provided by the Hartree-Fock-Bogoliubov approach
at finite temperature. In this framework we analyse the dependence of the crust
thermalisation on pairing properties and on cluster structure of inner crust
matter. It is shown that the pairing correlations reduce the crust
thermalisation time by a very large fraction. The calculations show also that
the nuclear clusters have a non-negligible influence on the time evolution of
the surface temperature of the neutron star.Comment: 7 pages, 5 figures, submitted to Phys. Rev.
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